Colorectal (CRC) cancer is the third most common cancer in the United States and its incidence is rising in younger populations. Diet is an important risk factor for CRC and dietary constituents are modified by gut microbial action. Microbial fermentation of dietary fiber and plant lignans in high-fiber foods produces bioactive endproducts, such as butyrate and enterolignans. The enterolignans, enterolactone (ENL) and enterodiol (END) have been associated with lower CRC in some epidemiologic studies. Lignans and dietary fiber have been shown to reduce colon tumors in animal models and butyrate and END and ENL influence cellular pathways important to cancer risk in vitro. We propose a 3-period randomized, cross-over intervention in 70 healthy men and women (ages 20-45) with the goal to test the effect of a flaxseed lignan supplement and defatted flaxseed meal (containing lignans + dietary fiber) as compared to placebo on: 1) host gene expression in epithelial and stromal cells from colon biopsies and exfoliated colonocytes extracted from feces; 2) gut microbial community composition, and 3) the interaction of the gut microbiome, enterolignan exposure, and colonic gene expression in high- and low-ENL excreters. Colon biopsies, stool, and urine will be collected at the end of each of the three 60-day periods to evaluate effects of the lignan treatments. We will use an innovative application of RNA-seq to measure gene expression in human colon cells from biopsy and stool. The gut microbiome will be characterized using pyrosequencing and QPCR of the 16S rRNA gene. Lignans will be measured in urine by GC-MS. Further, using samples from a subset of intervention participants who are low-and high-ENL excreters, we will measure the functional metagenomics of the gut microbiome in vitro. This proposed project will be the first to integrate and characterize, through a unique interdisciplinary collaboration, the relationships between lignan exposure, gut microbial ecology, and gene expression in cell- signaling pathways. Results of this placebo-controlled intervention will bridge the current knowledge from pre-clinical animal models and epidemiologic studies and will help to inform approaches for CRC prevention.